I assume that you saw challenges/issues that led you to get him evaluated in the first place -- he's still the same sweet boy he was before diagnosis, only now you have a roadmap and plan in place to help him.
I see that you're not planning on going to public next year, so I don't really see the value in contacting the public school district. They aren't going to offer you much of anything. I would question, though, whether montessori is the right fit for him, especially if he's having trouble at school. My child with ADHD needs more structure than montessori can provide.
Also, for your sticker chart, I assume you are defining "good day." Otherwise, it's too amorphous to help him make good choices. I would choose no more than three things he is struggling with -- so maybe a sticker for using his words when he's upset (instead of crying or hitting or whatever he does) and another sticker for joining the group at circle time. Or whatever skill you are targeting.